1. Field of the Invention
The present invention relates generally to the field of the protection of value documents against forgery. Particularly the invention relates to the protection of value documents against forgery involving heat sensitive erasable ink compositions and the use of pens comprising a refill containing such heat sensitive erasable inks.
2. Discussion of Background Information
The commercialization of a new type of ball point pens comprising an ink refill containing heat sensitive erasable ink, also called in the patent literature Thermochromic Coloring Color-Memory Composition, represents a potential threat in the field of security documents.
Heat sensitive erasable ink compositions, in particular heat sensitive erasable aqueous ink compositions, are disclosed, for example, in EP 2 138 550, U.S. Pat. Nos. 4,720,301 and 7,494,537, US 2009/0050013 A1, US 2010/0120614 A1, US 2011/0008095 A1, EP 1 820 662, EP 2 072 277, JP 2010-241867 and JP 2010-229333, the entire disclosures of which are incorporated by reference herein.
Ball point pens containing a refill containing heat sensitive erasable aqueous inks are now widely commercially available (e.g. FriXion™ Ball pens from Pilot or uni-ball Fanthom™ pens from Mitsubishi). The writing implement comprising the heat sensitive erasable ink formulation have been disclosed for instance in US 2011/0008095 A1, the entire disclosure of which is incorporated by reference herein.
The specific properties of a heat sensitive erasable ink are disclosed e.g. in U.S. Pat. No. 4,720,301. The heat sensitive erasable ink composition comprises, as the main components, an electron-donating organic dye, a phenolic compound and an ester compound. The electron-donating organic dye acts as base; its protonated form is coloured while its unprotonated form is colourless. The phenolic compound acts as acid. The ester compound acts as a matrix for the acid-base pair. The ester compound is characterized by its melting and cloud points which are comprised in a range between 5° C. and 50° C. The specific properties of the heat sensitive erasable ink are imputed to the large thermal maximum hysteresis between the different coloured states of the ink.
Indicia or texts written with a heat sensitive erasable aqueous ink may become discoloured by thermal treatment of the indicia: the ink forming the indicia is not physically removed from the substrate; however, through a thermal treatment, the written indicia become invisible to the naked eye of an observer. The thermal treatment may be produced by a heat source, e.g. an open flame or a hair-drier; or alternatively the thermal treatment of the indicia may simply consist in a mechanical rubbing of the ink layer, for instance with a conventional eraser or with a piece of synthetic rubber. The commercialized ball point pens comprising heat sensitive erasable ink refills are equipped with a rubber tip useful to render invisible (“to delete”) written indicia or text.
The ink refills of the commercial pens comprising heat reversible thermal discoloration ink (i.e. heat sensitive erasable ink) are removable and refills are commercially available separately. Thus, a forger may insert an ink refill containing a heat sensitive erasable ink composition, for instance a heat sensitive erasable aqueous ink composition, into an ordinary or a luxury pen and urge a person to use this pen to fill in a value document, for instance a bank cheque; the forger could then easily alter the hand-written information.
In view of the foregoing facts such ball point pens containing a heat sensitive erasable ink formulation and the corresponding refills present a threat in the field of security documents. Forgers may use them for the falsification of value documents, for instance of identity documents, bank cheques, vouchers, fiduciary acts, deeds or certificates.
It is known in the art that value documents, such as for instance cheques, are protected against forgery or tampering attempts through the use of special inks for the background printing. In particular, specific inks have been developed to coat with a printed layer the background of the value document or cheques, in particular in the document areas that must be completed with hand-written information: thus the indicia or the text hand-written in these areas cannot be fraudulently altered or deleted. Usually tampering attempts are performed by using solvents or acids to dissolve the ink of hand-written indicia; or alternatively forgers attempt to delete the information with a conventional eraser. By using specific inks to print the documents background, any tampering attempts leave a clearly visible mark on the document. Specific inks used for this purpose are for instance solvent reactive inks, water fugitive inks or mechanically erasable inks. The protection of the security documents relies upon a colour fading, a colour change or disappearance, or a bleeding off when the document is subject to a falsification attempt by any means such as e.g. an eraser, an organic solvent or an aqueous solution. Fugitive inks are known in the art (see e.g. The Printing Ink Manual, Ed by R. H. Leach and R. J. Pierce, 5th edition, page 341. EP 0 835 292, the entire disclosure of which is incorporated by reference herein, discloses for instance such a solvent bleeding ink composition used to print value documents.
However, neither solvent reactive inks nor water fugitive inks would prevent indicia written with a heat sensitive erasable ink to be deleted or rendered invisible through a thermal treatment.
Thus there is a need for a new ink composition, in particular among printers manufacturing value documents, for instance identity documents, bank cheques, vouchers, fiduciary acts, deeds or certificates or other value documents. The new ink composition should combine the forgery prevention properties of the known fugitive ink compositions with new properties aimed at preventing forgery by means of heat sensitive erasable inks.